A standard horn antenna, as the name suggests, is a radio antenna having the general shape of a horn. The horn antenna produces an electric (E) field that is uniform in the direction of the E field and has constant amplitude across its radiating aperture in that direction. A problem that the uniform field that a standard horn antenna produces is high side lobe levels in the E plane of the electromagnetic field that emits from the antenna. Energy goes into and is needlessly expended in the side lobes. This side lobe energy is wasted and, in some applications, can interfere with main lobe performance. As such, they are generally undesirable. To make the side lobes as small as possible, therefore, reduces energy requirements and eliminates interference for the electromagnetic fields from the antenna.
Some horn antennas use a parabolic reflector to direct or use the energy more efficiently. With a parabolic reflector, however, the side lobes problem still exists. Side lobes that miss the reflector produce unwanted response in directions outside the main lobe and waste energy. The side lobes also cause undesirable main lobe patterns when they hit the parabolic reflector. It is desirable, therefore, to minimize side lobes levels from a horn antenna. The only known way to minimize side lobe levels is to provide a cosinusoidal electromagnetic field distribution in both the electric or E field plane and the magnetic or H field plane.
Known methods to produce a cosinusoidal electromagnetic field distribution are to use a corrugated horn antenna or a scalar feed antenna to taper the electric field portion of the electromagnetic field. Tapering produces side lobes that are approximately 17 to 18 dB down from the peak. Both the corrugated horn antenna and the scalar feed antenna rely on 1/4 to 1/2 wavelength deep corrugations for operation. The corrugated horn antenna has 1/4 wavelength corrugations that are cut into the inside wall of horn portion surface in the E plane. This prevents currents from flowing in the horn portion surface. The corrugations present, in essence, an impedance to the surface current that causes the electric field to have a cosinusoidal distribution.
The scalar feed antenna has a circular feed configuration with corrugations built into a radio ground plane that encircles the antenna. The corrugations in the scalar feed antenna are also 1/4 wavelength to 1/2 wavelength deep. Both the corrugated horn antenna and the scalar feed antenna have the limitation of relying on the 1/4 or 1/2 wavelength corrugations. Because of this limitation, operating a conventional horn antenna outside of its design range no longer provides the desired impedance to the electromagnetic field. This can cause main lobe pattern deterioration as well as produce the undesirable side lobe patterns that radiate from the antenna. These effects are both undesirable and uncontrollable.
Known horn antennas are also bulky, of narrow bandwidth, and difficult to build. The wall of the corrugated horn antennas must be thick enough to physically support the 1/4 wavelength corrugations that are typically on the order of approximately three inches thick for a radio frequency of 1 GHz. For most of these antennas, a 2:1 bandwidth is the absolute maximum possible bandwidth. These antennas are also hard to build because they must be built in pieces or segments. It is necessary to machine the horn walls, assemble them, and weld them together. In addition, the corrugations must be precisely formed and assembled. If the corrugations are not precisely spaced or of a precise depth, a less than optimal lobe pattern may result.
Another previous problem related to known horn antennas is the fact that an impedance match or minimal voltage standing wave ratio (VSWR) is difficult to achieve. With the corrugated horn antennas, power is reflected back into the antennas where the horn portion starts. This occurs because the electromagnetic wave sees at this point an immediate transition from smooth wall of the antennas wave guide to corrugated wall of the horn portion. A similar problem occurs with the scalar feed antennas.